Amplifier device



Dec; 19, 1939. KREHENFELD 2,183,748

AMPLIFIER DEVI CE Filed llay 22. 1936 INVENTOR F. K re fen fe/d ATTORNEYPatented Dec. 19, 1939 AMPLIFIER DEVICE Friedrich Kreienfeld, Berlin,Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphicin. b. H., Berlin, Germany, a corpo ration of Germany Application May22, 1936, Serial No. 81,203 In Germany July 19, 1935 2 Claims.

The present invention aims at preventing oscillations of areturn-coupled tube stage when its coupling to a second circuit isvaried.

If the coupling of the input circuit of an ampli- 5 fier tube with thesource of signal currents is varied for the purpose of controlling thevolume, I have found that when the usual type of feed back circuit isused, a reduction of the coupling below a certain value causes theamplifying stage to become unstable and commence oscillating.

This difficulty can be overcome by so limiting the intensity of the feedback that oscillations do not occur even when adjustment is made for theloosest coupling. For a fixed coupling it should however be understoodfrom the beginning that I no possibility exists of utilizing the entirefeed back available.

These drawbacks can be overcome through manual control of the feedback,however, this complicates the adjustment considerably, since besidesadjusting the tuning and the coupling it will also be necessary toadjust the feed back. Furthermore, the servicing is rendered still moredifficult in view of the fact thatthe adjustments are in partindependent of each other.

According to my invention, the said difiiculties are overcome in that apart of the feed back turns are arranged on the turnable coupling coilin the preceding circuit. In this way, upon reduction of the couplingbetween the two circuits, the

amount of feed back will be automatically reduced.

For a better understanding of the invention reference is made to theaccompanying drawing showing a circuit arrangement embodying the same.The radio frequency amplifier R is shown as having a resonant inputcircuit H tunable to the signal frequency and comprising a fixedsecondary coil C tuned by the variable condenser D.

The antenna circuit A contains the adjustable or rotatable primary coilB coupled to C, the degree of coupling varying with the adjustment ofcoil B. The feed back circuit between the plate I0 and grid circuit Hcontains the feed back coil F which is rotatable or adjustable withreference to coil C, and also the coil S which is fixedly mounted onantenna coil B. Thus, if the coupling between the coil B and coil C ofthe input 5 circuit is decreased by adjustment of B, by means of thearrangement described the value or amount of feed back will be decreasedat the same time and the tube prevented from going into an oscillatingcondition.

Having described my invention, what I claim as novel and desire tosecure by Letters Patent is:

1. In an amplifying device, the combination of a vacuum tube having ananode circuit and a resonant grid circuit, said resonant grid circuit 15including a transformer secondary winding, an antenna circuit includinga primary winding adjustably coupled to said secondary winding, and afeedback circuit connected between said anode circuit and said resonantgrid circuit, said feed- 20 back circuit including two series connectedwindings, one of said windings being adjustably coupled to saidtransformer secondary winding, the other of said series connectedwindings being mounted in fixed relationship with respect to said 25primary winding and coupled to said secondary winding wherebyadjustments in the coupling between the secondary winding and theprimary winding also afiect the coupling between said secondary windingand said last named series 30 connected winding. 2. In an amplifyingdevice, the combination of a vacuum tube having a resonant grid circuitincluding a transformer secondary winding and an anode circuit, arotatably mounted primary wind- 35 ing adjustably coupled to saidsecondary winding, and a feed back circuit coupled to said anode circuitand including a first coil having a fixed degree of coupling with saidprimary winding and a second coil adjustably coupled to said sec-- 4.0ondary winding.

FRIEDRICH KREIENFELD.

